1. Field of the Invention
The invention relates to a method of producing a steel component provided with a metallic coating giving protection against corrosion, by the forming of a flat steel product composed of an Mn-containing steel which is provided with a coating of ZnNi alloy before the forming of the steel component.
2. Description of Related Art
When in the present case “flat steel products” are mentioned, what are meant by this term are steel strips, steel sheets, steel plates, or blanks and the like obtained therefrom.
To give the combination of low weight, maximum strength and a protective action which is called for in the construction of modern-day vehicle bodywork, what are currently used in areas of the bodywork which may be exposed to particularly high stresses in the event of a crash are components which are formed from high-strength steels by hot pressing.
In hot press hardening, steel blanks which are taken from hot or cold rolled steel strip are heated to a forming temperature which is generally above the austenitising temperature of the given steel and when in the heated state are placed in the die of a forming press. In the course of the forming which is then carried out, the blank of sheet or plate material, or rather the component formed therefrom, undergoes swift cooling as a result of the contact with the cool die. The cooling rates are set in this case in such a way that a hardened microstructure results in the component.
A typical example of a steel which is suitable for hot press hardening is the one known by the designation “22MnB5” which can be found in the Steel Key (Stahlschlüssel) for 2004 under the material number (Werkstoffnummer) 1.5528.
In practice, the advantages of the known MnB steels which are particularly suitable for hot press hardening are offset by the disadvantage that manganese-containing steels are generally not resistant to wet corrosion and are difficult to passivate. The corrosion concerned, though only local, is heavy, and the tendency for it to occur when exposed to elevated concentrations of chloride ions is high in comparison with less highly alloyed steels and this tendency makes it difficult for steels belonging to the category of materials known as high-alloy sheet steels to be used in the very field of vehicle bodywork construction. Manganese-containing steels also have a tendency to area corrosion, which is likewise a factor which restricts the range of uses which can be made of them.
The search is therefore going on for possible ways of providing manganese-containing steels too with a metallic coating which will protect the steels against corrosive attack.
In the method of producing components by hot press hardening which is described in EP 1 143 029 B1, a steel sheet or plate is first to be provided for this purpose with a zinc coating and then, before being hot formed, is to be heated in such a way that, in the course of the heating, an intermetallic compound comes into being on the flat steel product as a result of a transformation of the coating on the steel sheet or plate. This intermetallic compound is intended to protect the steel sheet or plate against corrosion and decarburizing and to perform a lubricating function during the hot forming in the pressing die.
A wide variety of problems have become apparent when attempts have been made to implement in practice the procedure which is proposed in a general form in EP 1 143 029 B1. In this way, it has proved to be difficult for the zinc coating to be applied to the steel substrate in such a way that, once the intermetallic compound has formed, it can be guaranteed that the coating will adhere sufficiently well to the steel substrate, that the coating will have adequate coatability for a paint finish to be applied subsequently and that both the coating itself and the steel substrate too will have adequate resistance to the formation of cracks in the course of the hot forming.
A proposal as to how zinc coatings to which an organic coating can be applied particularly well can be produced on steel strips is described in EP 1 630 244 A1. Under this proposal, a layer of Zn containing up to 20 wt.-% Fe is applied to the steel sheet or plate to be processed either electrolytically or by the use of some other known coating process. The steel sheet or plate which has been coated in this way is then heated from ambient temperature to 850-950° C. and is formed by hot pressing at 700-950° C. What is mentioned as particularly suitable for the production of the layer of Zn in this case is electrolytic deposition. In this known method, the layer of Zn may also take the form of a layer of alloy. What are cited in EP 1 630 244 A1 as possible alloy constituents for this layer are Mn, Ni, Cr, Co, Mg, Sn and Pb and Be, B, Si, P, S, Ti, V, W, Mo, Sb, Cd, Nb, Cu and Sr are also mentioned as additional alloy constituents.
Something that is essential to the method described in EP 1 630 244 A1 is that the 1-50 μm thick Zn coating which is present on it comprises an iron-zinc solid solution phase and has a layer of zinc oxide whose thickness is restricted, on average, to not more than 2 μm. What is done for this purpose in the known method is either that the annealing condition at the time of the heating to the temperature required for the forming by hot pressing is selected to be such as to produce, at least, a controlled formation of the oxide, or that, after the hot forming, the layer of oxide present on the steel component obtained is at least partly removed by a machining or particle-lifting process sufficiently for the oxide layer to be kept to the maximum thickness given in EP 1 630 244 A1. Hence, this known procedure too calls for costly and complicated measures on the one hand to ensure that the Zn coating will have the desired anti-corrosive effect and on the other hand to ensure that the good coatability and adhesion for paint which are required will exist in a painting operation which takes place after the hot forming.
Known from DE 32 09 559 A1 is a further method by which a coating of zinc-nickel alloy is deposited electrolytically on strip steel. In the course of this method, the strip to be coated is subjected, before the ZnNi coating is deposited, to intensive non-electrical pre-treatment to produce on it a thin primary layer containing zinc and nickel. Following this the actual zinc-nickel coating is then applied electrolytically. So that the electrolytic deposition of the alloy coating is constantly performed with a preset composition, separate anodes are used which each contain only one alloying element. These anodes are connected to separate circuits to enable the current flowing through them, and hence the release of the given metal into the electrolyte, to be set in a targeted way.
The results of a systematic examination of the properties of zinc alloy coatings on a steel sheet which was composed of a hardenable steel are given in WO 2005/021822 A1. The coating was composed in this case essentially of zinc and contained in addition one or more elements with an affinity for oxygen in a total quantity of 0.1 to 15 wt.-% as a percentage of the coating as a whole. What are actually cited in this case as elements with an affinity for oxygen are Mg, Al, Ti, Si, Ca, B and Mn. The steel sheet which had been coated in this way was then raised to a temperature required for hardening while atmospheric oxygen was admitted. In the course of this heat treatment, a surface layer of oxide of the element or elements with an affinity for oxygen was formed.
In one of the trials which are described in WO 2005/021822 A1, a ZnNi coating was produced by the electrolytic deposition of zinc and nickel on a metal sheet of unspecified composition. The ratio by weight of zinc to nickel in the anti-corrosion layer was approximately 90:10 for a layer thickness of 5 μm. The sheet which had been coated in this way was annealed for 270 s at 900° C. in the presence of atmospheric oxygen. This produced, as a result of diffusion of the steel into the layer of zinc, a thin diffusion layer composed of zinc, nickel and iron. At the same time, the bulk of the zinc oxidised into zinc oxide.
From the findings which are documented in WO 2005/021822 A1 it is evident that the ZnNi coating obtained in the above way provided pure barrier protection and did not have any cathodic anti-corrosion effect. Its surface was of a scaled, green appearance with small local areas of peeling where the layer of oxide did not adhere to the steel. According to WO 2005/021822, the reason for this was that the coating itself did not contain an element with a sufficiently high affinity for oxygen.